In a long-awaited fusion between hot-blooded hormones and cold-headed engineering, a Japanese lingerie company has produced a bra they claim will only unlock when the wearer is really in love.

The "True Love Tester" uses sensors and a special gadget linked to a mobile device to analyse the pattern and speed of the user's heart beat in the heat of the moment.

If the app determines the woman's feelings are for real, the clasp at the front pops open to allow matters between lovers to take their course.

The bra is a gimmick by lingerie brand Ravijour, which says it is not for sale, but forms part of a publicity campaign for the marque's 10th anniversary.

"We wanted to do something that wouldn't just appeal to people who wear our products, but also to lift the romantic mood between men and women," spokeswoman Yuka Tamura said.

In a promotional video, viewers are shown how the glittering gold lame bra conceals sensors placed inside the cup, which send wireless signals to a smartphone.

The app studies the heart's changing pattern and the duration of the change, which the makers say will allow it to distinguish between activities as diverse as jogging and flirting, where a woman's requirements of her bra are quite distinct.

In a promotional video clip, a toned model stands in a dimly lit studio wearing only her underwear, which keeps the insistent alpha male at bay because he cannot unhook the clasp.

It is "a revolutionary bra that knows truly how women feel," the video says.

The video can be seen at: https://www.ravijour.com/anniversary/moodup